Already a century ago it has been proposed (see German Pat. No. 13,364 of 1880) to use an upper and a lower transporter for separately displacing two fabric layers to be stitched, each transporter being designed as an intermittently operable feed dog whose motions are synchronized with the needle motion and which clamp the two layers between them. By changing the transport stroke of one of these feed dogs, specifically the upper one, a desired amount of excess material can be introduced in the corresponding fabric layer. Since the stroke adjustment of such a transport mechanism can be performed only on standstill, a speed difference once selected must be maintained as long as the machine is in operation. Subsequent improvements have included the provision of a scale for measuring the transport stroke, e.g. as described in German Pat. No. 975,242 (1961).
Even with precise presetting, however, unavoidable differences in the movable masses of the two intemittently operating transporters result in cumulative deviations which may add up to undesired offsets between the two layers. A continuous intervention of the operator has therefore been hitherto necessary to maintain the desired relationship between the rates of advance of these layers. Such intervention, however, is not practical in plants with large numbers of automatic high-speed sewing machines in which the task of the operators is limited to an initial positioning of the workpieces and their removal from the machines.